Method of operating reduction-melting furnaces



Dec. 24, 1946.

Filed Nov. 5, 1943 & N

J. E. CARTER ETAL Y METHOD OF OPERATING REDUCTION-MELTING FURNACES.

(595a) syruuusd 31. #003 30 D 7 Sheets-Sheet 1 9 ID NUMBER OF HOURS HFTER FINISHING CHHRG/NG c O 16 u IN VENTORS ZHEHdSOI VLH JDVNHDJ N/ NEIDOUOAH JOSEPHEDW/N CARTER RAY KERM/ ENSLER l TTORNEY 1945 J. E. CARTER ET AL 2,413,215

METHOD OF OPERATING REDUCTION-MELTING FURNACES Filed Nov. 5, 1943 7 Sheets-Sheet 2 o 930) swuua .4 Joey say/var Q 0) 3 4 NUMBER OF HOURS HFTER FINISHING CHfiRG/NG JOSEPH EDWIN CARTER- RAY/TERM/T GENSLER HTTO R/VEY Dec. 24, 1946.

J. E. CARTER ET AL Fi led Nov. 5, 1943 7 Sheets-Sheet 3 920) 80458341 JOOH o: .L/vsaasa su au3Hdspwa soy/van: N/ N3900AH HVVENTORS V JOSEPH EDWIN CARTER RAY HER/WIT GENSLER a. WM

flTTOR/VEY Dec. 24, 1946. J. E. CARTER ET AL 5 METHOD OF OPERATING REDUCTION-MELTING FURNACES Filed Nov. 1943 7 Sheets-Sheet 4 (bread) PHILLHHHdM/BL .4008 sauNHflJ 6 7 NUMBER OF HOURS HFTER FINISH/N6 CHflRG/NG m ATTORNEY Dec. 24, 1945. CARTER 2,413,215

METHOD OF OPERATING REDUCTION-MELTING FURNACES Filed Nov. 5, 1945 7 Sheets-Sheet 5 (393a) SHOJHUHdWS-L .daoa sowvund 6 9 NUMBER OFHOURS HFTER FINISH/N6 CHfiRGl/VG OD .LN3 983d SE a u 39bN/fid 1w lvssouaxu gHdgo/ML JOSEPH EDWIN CARTER RAY KERM/T GENSLER HTTOR/VE) INVENTOFrS Dec. 24, 1946. J. E. CARTER EIAL. 2,413,215

v METHOD OF OPERATING REDUCTION-MELTING FURNACES g E Q 0 B v k .dOOH EDUNU/N O-L FLUE INVENTORS JOSEPH ED W/N CARTER RAYKERM/T GENSLE'R ATTORNEY Dec. 24, 1946. J CARTER AL 2,413,215

METHOD OF OPERATING REDUCTION-MELTING FURNACES Filed Nov. 5, 1943 7 Sheets-Sheet 7 BACK 7 INVENTORS JOSEPH EDWIN CARTER RAY KER/WIT GENJL ER (3 .(kW

' HTTORNE'Y W n j c m 1 R m mm m m m R M I A a w T II E m W Mm :w u

Patented Dec. 24, 1946 METHOD OF OPERATING REDUCTION- MELTING FURNACES Joseph Edwin Carter, Huntington,

and ,Bay

Kermit Gensler, Guyandotte, W. Va., assignors to The International Nickel Company, Inc., New York, N. Y., a corporation of Delaware Application November 5, 1943, Serial No. 509,154

7 Claims.

The present invention relates to a method for operating reduction-melting furnaces in which an oxide-containing material is melted down in admixture with a reducing agent to produce molten metal and more particularly to .the reductionmelting of nickel oxide.

In the operation of furnaces, such as annealing furnaces and furnaces employed for melting charges which consist of metallics, the control of the atmosphere of the furnace can be readily obtained. This control is based upon the use of a fuel of relatively constant composition and an application of the well known water gas equation providing an equilibrium constant at any particular temperature for the composition of the furnace atmosphere dependent upon the relationship between carbon dioxide, carbon monoxide, water and hydrogen which is expressed by the following equation:

In the operation of the aforesaid types of furnaces in which the only factors affecting the furnace atmosphere are the composition of the gaseous fuel, i. e., air/gas, and the temperature (when the composition of the gaseous fuel is constant), determination of the condition of the furnace atmosphere can be readily made by means of automatic instruments, principally of two types. One type is the so-called specific gravity type of instrument in which the specific gravity of a sample of the furnace atmosphere and the specific gravity of a standard gas, usually air, are determined under the same conditions of temperature and moisture content of the gases, and the specific gravities automatically compared. The other type is the so-called therma1 conductivity type of instrument which is based upon the difference in thermal conductivities of different gases. In this type of instrument a carefully calibrated platinum wire is heated by means of a constant electric current. The gas to be tested is then made to flow through the tube containing the carefully. calibrated wire and the difference in the resistance in the hot wire is measured. As has been pointed out hereinbefore, the atmosphere of annealing furnaces and furnaees employed for melting charges consisting of metallics can be readily controlled by means of instruments of either of the foregoing types. However, many attempts have been made to control the furnaceatmosphere of furnaces employed for melting charges in which a reducing or oxidizing reaction likewise takes place between constituents of the charge. .Under these conditions, although the composition of the gaseous fuel may be relatively constant, repeated attempts to employ either the specific gravity type or the thermal conductivity type of automatic control innace probably are entirely different.

struments have metwith failure. .It can be readily appreciated that, in the melting of a charge containing for example nickel oxide whichis to be reduced by coke present inthe furnaccharge, the reaction between the coke of the charge of the nickel oxide of .the charge will have an appreciable effect upon the equilibrium of the furnace gases due tothe production of carbon monoxide and carbon dioxide by the reactionof the. nickel oxide and the coke of they charge.

For many years it has been thepracticeto .conduct the.reduction-meltingof nickel pigs and the like with the addition of cokeor-similar carbonaceous reducing agent in furnaces heated .with natural gas which not only provides the fuelfor heating the charge, but also provides .ailarge amountof carbon'for reducing the ox gencom tent in the charge. In thepast, control of these furnaces to provide the most fiicient operation thereof has been provided by obtainingpsfamples of .the furnace atmosphere and .ganalyzin'g these samples individually to determine therc'arbon monoxide content of the'furnace; gas. However, it was recognized by thoseskilled inthe art that r' the operation of analyzing thegasfforicarbon monoxide required a sufiicient timeeven" in ,the hands of expert technicians that the reported carbon monoxide content of the furnace atmosphere was merely a .spot determination which might or might not truly represent the condition of the furnace atmosphere when reported later. [In other words, in the hands of an experttech nician, the determination of the carbon monoxide content of the furnace gas could only be made at the rate of once in about forty-five minutes; As a consequence, the furnace operator actually knew the composition of the furnace atmosphere at the end .of each three-quarters ,of an hour after the furnace atmosphere had been sampled.

It was also recognized that on many occasions the cave-in occurs to a point indicating that the furnace atmosphere is too rich for .eincient operation. However, by' thetimetthe report was turned over t'o-the operator, conditions in the"fur= Further more, since the analyses of the gas samples are usually made by the same technicianduringthe entire shift, certain errors due to the personal ing furnaces in which the carbon monoxide content of the furnace gas is maintained within critical limits by means of operation in accordance of reducing agent which reacts with the oxide of the nickel pigs during a unit of time. In addition, it has been discovered that the efficient operation of the reduction-melting furnace is dependent to a very great extent upon the composition of the furnace atmosphere. That is to say, if the furnace atmosphere is too rich, a smaller amount of metal will be melted in a unit of time than under a practically ideal condition. Furthermore, if the furnace atmosphere is too lean, the amount of metal melted in a unit of time will likewise be less than under satisfactory conditions. The furnace atmosphere itself is affected principally by three factors, (1) primary air-gas mixture introduced from the burners; (2) extraneous or secondary air introduced around burner parts through cracks in the furnace wall, etc.; and (3) the action of the carbonaceous reducing material as it reacts with or burns out of the oxide. Careful investigation of the operation of reduction-melting has shown that most of the reduction of the oxide occurs in the first sixteen hours of the heat and that during this period control of the furnace atmosphere is most critical.

Although the temperature of a furnace fired by means of a gaseous fuel having a substantially constant composition should provide a means for determining the composition of the furnace atmosphere from a consideration of the well known watergas equation, nevertheless it is possible to operate a furnace, such'as an open hearth furnace, for the reduction of nickel oxide and melting of the resulting nickel in such a manner that the temperature curve of an unsatisfactory cycle is practically the same as the temperature curve of a'satisfactory cycle. Thusit will be manifest that the unmodifiedapplication of the well known Water gas equation does not provide a satisfactory solution to the problem involved in operating a furnace for the reduction of nickel oxide and the melting of the nickel'produced thereby. However, it has been discovered that the irregularities in furnace operation from day'to day, as controlled by" means of Orsat determinations, or when controlled by means of instruments of the thermal conductivity type, or the specific gravity type, 'can beeo'vercome in an efificacious manner by application of theprinciples of the present invention. It has been discovered that the fur-' nace atmosphere can be analyzed automatically and continuously and the operation of the reduction-melting furnace controlled in accordance therewith. By means of the automatic and continuous determination of a constituent of the furnace atmosphere the time lag introduced by the inherent difficulties of the method dependent upon the Orsat determinations can be overcome with vastly improved operation of the furnace from a technical standpoint and with consider able economies in-man power and the like. 1 It is an object of the present invention to provide a method for operating reduction-melting furnaces in which nickel pigs containing oxide are melted and reduced in the presence of a carbonaceous-reducing agent and in which the operation is controlled in accordance with the hydrogen content of the furnace atmosphere as determined by an automatic analyzer, and reported in termsof per cent carbon monoxide.

It is another'object of the present invention to provide a processforoperating' reduction-meltwith an automatic analyzer.

It is a further object of the present invention to provide a process for the reduction-melting of nickel pig whereby consistently satisfactory melting and reduction is obtained through control of the furnace in accordance with carbon monoxide content of the furnace atmosphere as determined automatically and continuously.

The present invention also has as an object to provide an apparatus for controlling the operation of furnaces employed for the reductionmelting of nickel pig whereby consistently satisfactory meltin and reduction is obtained through control of the furnace atmosphere in accordance with the hydrogen content thereof expressed as per cent carbon monoxide.

Other objects and advantages will become apparent from the following description taken in conjunction with the drawings in which- Figure 1 is a chart illustrative of the temperature and corresponding hydrogen content (expressed as per cent carbon monoxide) of the furnace atmosphere during the reduction-melting of a nickel pig charge when the furnace atmosphere is too rich;

Fig. 2 is a chart illustrative of the temperature and corresponding hydrogen content (expressed as per cent carbon monoxide) of the furnace atmosphere during the reduction-melting of a nickel pig charge when the furnace atmosphere is too lean;

Fig. 3 is a chart illustrative of the temperature and corresponding hydrogen content (expressed as per cent carbon monoxide) of the furnace atmosphere during the reduction-melting of a nickel pig charge when the furnace atmosphere is well controlled;

Fig. 4 is a graph illustrative of the graphic band indicating hydrogen content (expressed as per cent carbon monoxide) of furnace atmosphere at various times to provide a satisfactory melting and reduction of nickel pigs;

Fig. 5 is a graph illustrative of variation in temperature and hydrogen content (expressed as per cent carbon monoxide) during the operation of a reduction-melting furnace treating nicke1 pig in which the control of the furnace was maintained in accordance with the principles of the presentinvention and the hydrogen content (expressed as per cent carbon monoxide) of the furnace atmosphere maintained within a graphic band;

Fig, 6 is illustrative in a more or less diagrammatic manner of an apparatus for automatic sampling and analyzing of the atmosphere of a reduction-melting furnace for the determination of hydrogen content (expressed as per cent carbon monoxide); and

Fig. 7 illustrates more or less in detail an apparatus suitable for determining the hydrogen concentration of furnace gases by the specific gravity method and reporting thesame in terms of carbon monoxide concentration. 7

A consideration of the operation and the reactions which occur in a reduction-melting operation involving the melting and the reduction of nickel pigs containing nickel oxide will assist in understanding the present invention. The charge to the furnace, which preferably .is of the open hearth type, may consist, for example,

ofabout 60,000 pounds of nickel oxide and about 12% or about 7200 pounds of coke. The nickel oxide and .the coke are thoroughly mixed, pr.eferably'by mechanical .means, .until a substantially homogenous mixture is obtained. IThewmixture islthen introduced into the furnace preferably in such a manner as toprovide two conically shaped piles which slope away toward the hearthas they approach the sides of the furnace, well below the level 'of the-burners. About 50% of the chargeprefera-bly is placed in the *conicalypile toward the front of the furnace and the-balance in the conical pile toward the back of the furnace.

ter the furnace has been charged, the gaseous fuel is'turned on immediately and the reduction-melting process begun. As a result of long practice, the amount of gas employed is pretty well standardized although it may be varied from time to time to meet changing conditions. In order to assure that the melting takes place at a normal rate, the operator watches the trendtof conditions within the furnace very carefully. 'From this point on, the control cf the atmosphere of :the furnace is extremely important. If the atmosphere is too rich, that is, the flue gases carry such a high content ofunburned combustibles that excessive amounts of heat are lost with them as they leave the furnaceand burn in the fines, the temperature of the furnace rises 'too slowly. On the other hand, if conditionsusually due to too rich firing, in

the furnace are such that low temperatures, as

indicated, by roof temperatures below 2650". F. to 2750" obtain in the 'furnacathe melting of the charge is-retarded. It has been found that melting is negligible in extent below roof temperatures of 2650 F. to 2750 1 Furthermore, if the furnaceatmosphere'is too lean, carbon will tend to burn out of the surface of the charge without completely reducing the oxide. Under these condition of too lean a fur nace atmosphere, a bright refractory layer of oxidized nickel is formed on the surface of the charge causing the phenomenon which is known among those skilled in the art as icing. This bright refractory layer of oxidizednickel reflects the heat away'from the unmelted charge and delays further reduction. ,Such a bright refractory skin or layer-of oxidized'nickel must be broken up by ;po1ingthe charge and adding more carbon when necessary. It has also been found advantageous, in order to maintain a uniform furnace atmosphere, to maintain a positive pressure within the furnace throughout the melting cycle. A'pressureof about 0.025 inch of water measured at the roof ofthe furnace is sufiicient for this purpose.

It-has beenfound that the difficulties encountered when too rich a furnace atmosphere obtains or when too lean a furnace atmosphere is produced can be overcome by controlling the hydrogen content of the furnace atmosphere (expressed as percent carbon monoxide) between certain critical limits, depending upon the length of time the charge has been heated. That is to say, most eflicient melting of the charge can be obtained duringthe first six to eight hours of thecycle by maintaining a hydrogen content (expressed as percent carbon monoxide) in the furnace atmosphere ranging preferably from about 6% to about 9.5% at the beginning-of the cycle downto about 1.5% to 2% to about 3.75% to 4% ateight hours. During the next two hours of the cycle, i. e., the eighth and ninth hours of the cycle, the hydrogen content of the furnace atmosphere (expressed as percent carbon monoxide) preferablyshould" be: maintained between about 1.8%.and.4% to about-l;2%.to about 3.6%; Those skilled in the art'will-understand athatvaris ationsiof about .0.1';% .to.about 0.15% may bepermitted lin the. upper and lower limits; set .fort herein.

IIn the'third portion of the .cycle, i.'.e., fromth ninth to the sixteenth. hours, the hydrogen'rcontento'fithe'furnace gasesiexpressed as percent carbon tmonoxide) should be maintained: fairly constant between the "limits of about 1.2%" to about 1.6%.

Under .these conditions, the melting vofithe charge takes placeatafmostefficient rateof about 2000 pounds per hour, andthe total cycle from charging the furnace to substantially complete reduction andmelting of thecharge requires. about twenty-threehours. When the furnaceatmosphere is too rich, the melting ratewilldrop to as low asabout I300'pounds per houryand the capacity of the furnace is reduced toabout 67%. On the-other hand; .if the furnace atmosphere :is too lean, the capacity of the furnaceis reduced to about 81%.

The foregoing can be readily appreciated :from a consideration of the following tabulation-in which the melting rate, in terms of pounds per hour, for furnace cycles in which the furnace-atmosphere was controlled within the limits of the graphic band. is compared with the melting rate, in pounds per hour, for furnace cycles :in which the atmosphere during the first sixteen "hoursmf the cyclewas either too rich or too lean:

Furnace atmosphere .Meltin'grate (0-16 hours) (per-hour) Pounds Well controlled 1,-950 Too-rich; l, 320 Too lean W l, .620

The eifect'of ,a rich atmosphere-upon the rate at 'which the temperature in the furnace rises will be readily appreciated by studying the graph (Fig.

l -I;t;is manifest-that-under conditions such as are indicated by hydrogen content in thefurnace atmosphere as indicated in Fig. -1 (in terms of percent carbon monoxide) ,the temperature rise in the furnace is very slow ascomparedto thetemperature rise-under well controlled conditions as illustrated by the. graph (Fig. 3).

. Fig. 2 reveals therapid temperature rise which is characteristic of lean furnace atmospheres but under :which the low-melting rate indicatesan unsatisfactory operation of the furnace, although more satisfactory than operation with too rich firing.

Fig. :3, which is indicative of conditions 'obtainingin well-controlled firing obtained by'controlling the furnace operation in accordancewith the automatic continuous determination of the hydrogen content of the furnace atmosphere within the limits of the"graphic band, shows a rapid attainment of temperatures exceeding-28.00 Fqduring'which good melting conditions Were obtained and likewise were maintained throughout the balance of the heating cycle.-

'Fig. 4 ;is illustrative of, the so-called graphic heavy continuous lines. outline the area within which. it is' preferred to maintain the hydrogen content ofzthe furnace atmosphqreduring various portions of the first sixteen hours of the cycle. The heavy lines are indicative of the limits within which the hydrogen content. (of the furnace atmosphere (expressed as carbon monoxide) may vary at different times during the initial portion of the furnace cycle. Those skilled in the art will understand that continued operation of the furnace under conditions in which the hydrogen content (expressed as per cent carbon monoxide) of the furnace gases approaches very closelythe limits indicated by the dotted lines, while better thanthe operations resulting from control in accordance with Orsat determinations, is not as satisfactory as operations in which thehydrogen content of the furnace atmosphere is maintained within the area delineated by the solid lines.

Fig, is illustrative of curves based upon isolated values read off from curves produced automatically and continuously during actual operation in accordance with the process of the present invention. The heavy solid lines indicate the desirable temperatures and the corresponding hydrogen contents of the furnace atmosphere during particular periods of the early portion of the furnace cycle while the discontinuous lines indicate the limits within which satisfactory op eration can be obtained. These curves are to be compared with corresponding curves in Figs. 1 and 2. Such a comparison will make manifest the much more uniform operation of the furnace which is obtained in accordance with the present process as compared with operation controlled in accordance with spot determinations of the carbon monoxide content of the furnace gases by means of the Orsat apparatus.

Fig. 6 is a schematic illustration of the equip' ment for carrying out the present process.

Fig, 7 is a more or less diagrammatic illustration of an apparatus suitabl for the automatic and continuous determination of the hydrogen content of the furnace gases, reported in terms of carbon monoxide, of a furnace for the reduction-melting of nickel pig whereby the operation thereof may be controlled in accordance with the present process. I

A consideration of the discussionprovided hereinbefore makes it manifest that the'capacity' of a given furnace can be appreciably increased by good control of the furnace atmosphere as indicated by the hydrogen content thereof. Such control is not possible as a day to day consistent operation when the furnace operator must depend upon spot sampling by hand, reports of which can only be made to the operator once in about an hour. a T J Accordingly, it will be appreciated that automatic determination of the furnace atmosphere provides a means whereby reduction-melting furnaces can be operated to consistently produce the maximum tonnage of which the furnace is capable.

Referring more particularly to Figs. 6 and 7, it will be seen that the furnace gases are sampled preferably through a water cooled sampling tube I of conventional structure placed approximately in th center of the furnace roof 2 at a point wher the furnace gases leave the furnace 3. The sample of gas is piped by means of conduit 4, having a water leg 5, down to the side of the furnace at the back where it passes through a drying unit 6. This unit consists of a container 1 of suitable size and may be of iron pipe about 3 inches in diameter and about 18 inches long equipped with the necessary connections. The container is filled with any suitable material 8 for the extraction of water from the furnace gases to provide a gas sample which is substantially dehydrated. It is preferred to employ Activated Alumina. It has been found that under cus-. tomary conditions of operation the drying unit should be replaced with a fresh unit once a week and, if desired, the old units reactivated and employed again. The substantially dry gas is then piped by means of conduit 9 to the instrument panel board where it passes through a plurality of carbon dioxide absorption units I0, preferable connected in series. The carbon dioxide absorbing units :0 preferably comprise a container ll filled with a carbon dioxide absorbent l2 and provided with appropriate connections. It has been found that flake caustic soda of the same commercial quality as used in pickling and cleaning metal sheets is satisfactory. Preferably each of these units holds about 8 pounds of flake caustic soda and with a flow rate of about 2 /2 cubic feet per hour of gas sample, the total consumption of caustic soda is about 4 pounds of caustic soda in 24 hours. While the carbon dioxide absorption units I0 may be mounted in any suitable position, it is preferable to mount the carbon dioxide absorption units in a horizontal position and in series. Difiiculty may be experienced with the units resulting from the absorbent packing down and caking to such an extent that the gas cannot flow through the unit. This difficulty, however, can be overcome by replacing the units with fresh units at the end of about 48 hours of use. The gas, substantially free from carbon dioxide, issuing from the absorption units In is then passed to a suitable means for comparing the specific gravity of a sample of the dried COz-fre furnace gas with a sample of the standard gas, under the same conditions of temperature and humidity. Since the furnace gas has been dried to a condition in which there is substantially no moisture present in the furnace gas, in order to eliminate blocking of g the conduit 9, the standard gas and the furnace gas sample are humidified to the same extent. A suitable instrument for comparing the specific gravity of the COz-free dehydrated furnace atmosphere with a standard gas is an instrument such as that manufactured in accordance with the disclosure provided by U. S. Patent No. 1,664,752 to Kiinig. The instrument is cali-' brated to show the percentage of carbon monoxide in the furnace gases, although the only gas in the dehydrated COz-free furnace atmosphere affecting the furnace instrument, as long as the furnace atmosphere is on the reducing side, is hydrogen. That is to say, the specific gravity of the furnace gas is primarily affected by the hydrogen content of the furnace gas when the furnace atmosphere is a reducing atmosphere. However, in view of the fact that during operation of furnaces such as described hereinbefore in ac-, cordance with control provided by Orsat determinations, the operators become so thoroughly accustomed to thinking in terms of carbon monoxide content that it is advisable to calibrate the instrument in terms of carbon monoxide rather than in terms of the hydrogen which is the constituent of the furnace'atmosphere which is actually "determined.

As those skilled in the art know, an apparatus for comparingthe specific gravity of an unknown gas with that of a standard gas, forexample air, such as. described in Konig 'U. 5, ,Patent No. 1,664,752, is provided with. two impeller fans; and IZ'WhiCh draw-samples of; air and test gas into twoso-called weighing chambers l3 and; I4. These impellers throw the respective gases against two impulse wheels [5 and I6 which develop opposing torques'in proportionto the densities of. the two gases being compared. These torques are reflected. in an equilibrium reading provided by an indicating pointer l! on a suitable scale I8.

The operation of a reduction-melting furnace in accordance with the principles of the present invention will be described in conjunction with the operation involving the reduction of nickel oxide and the melting of the metallic nickel so produced. An open hearth furnace is employed as will be readily understood by those skilled in the art from a consideration of the drawings in Fig; 6. The charge, consisting of about 60.000

pounds of nickel oxide and about 7200 pounds of coke, is introduced into the furnace in the usual manner. That is to say, an amount of reducing agent, such as coke, is employed which is equivalent to about 12% by weight of the amount of nickel oxide in the charge. The. nickel oxide and coke are intimately mixed and introduced into 7 the furnace. The fuel gas is preferably turned on immediately after charging and the reduction-melting process begins. For satisfactory operation of the furnace the operator must watch the trend of conditions within the furnace very carefully. It has been found that melting of such a charge is negligible when'the roof'temperatures are below about 2650 F. to about 2700 F. Furthermore, the uniformity of the furnace atmos-' phere is insured by maintaining; a positive pressure within the furnace (measuredat the roof) A pressure of throughout the melting cycle. about .015 inch to about .040 inch of water, and preferably of about .025 inch of water is sufficient to insure uniformity of furnace atmosphere. As soon as the heating of th charge ins, and at any rate shortly after-the charge has been heated for about an hour, the operator observes the fluctuation of the hydrogen content of the furnace (expressed as per cent-carbon monoxide) as indicated by the specificgravity determining apparatus. Although the gas employed in heating the furnace contains an amount of hydrocarbons equivalent" to about 20,000 pounds of carbon, it will be realized that the 7200 pounds of coke which are present in the chargeinfluence the character of the atmosphere of the furnace to a very great degree. In other words, the amount of carbon present in the charge is equivalent to about 40% of the carbon introduced into the furnace in the gaseous fuel;

nace gases (expressed as'p'er cent carbon monoxide) reaches a value between about 1.8% and about 4%. During the next two hours of--the cycle, thehydrogen content of the furnace gases (expressed as per cent carbon monoxide) should decrease still further, but at a considerably slower rate until about the ninth hour of the furnace cyclethe hydrogen content (expressed as per cent 10 carbon monoxide) should beabout 1.2% to'about 316%. After the ninth hour, the hydrogen content of the furnac gase (expressed as per cent carbonmonoxide) should beheld constant; at about 1.2% to about 3.6% for theremainder v of the melt.

By referring to Fig. 3, those skilled in the art will'readily appreciate. that when the operation of a reduction-melting furnace is controlled in accordance with the automatic continuous determination of the hydrogen content of the furnace gases, reported as per cent carbon monoxide, the temperature of the furnace-quickly reaches the. minimum temperature at which melting; of nickel may occur. That is to say, when the firing ofnthe furnace is controlled to provide-a furnace atmosphere having a composition are ported as between 6% and 9%% carbonmonox me, the temperature of the furnace asidetermined at the roof. reaches a temperature ofabout .2700 in about anhourand a quarter. On the other hand;- the graph of 'Fig. 2. clearly shows that with a lean-' atmosphere, the temperature of thefu'r' nace does notreach 2720'F. until about the end of two hours. Furthermore, when a rich atmos phere exists in the furnace; the furnace doesn'ot reach a temperature of about 2720 F. until very nearly seven hours after the start of the-reduc tion-melting cycle, Th graph of Fig. l lik'ewise isindicative of the dimculty'which is encountered in operating a furnace when the regulation of the furnace atmosphere is dependent for control upon determination of carbon monoxide made on spot samples and analyzed with an Orsat apparatus. A consideration of the curve marked' atmosphere will clearly show that throughout the operation the meltcr continued to try to maintain the furnace atmosphere as represented by the carbon monoxide content within -the g'raphicband, but altogether the carbon monoxide content was continually reduced, the operator was unable at any time to reduce it quite enough to bring it within the limits which have been found to provide satisfactory operation and maximum capacity for the furnace. Thus, it is manifest that when dependence for control is'placed upon spot analyses analyzed individually for carbon monoxide content, satisfactory efficient opera tion of a furnace for reduction meltingof oxidesis extremely difficult, whereason the othe'r'l'ra-nd; as can be clearly seen from a'consi'deratioii of Figs. 3 and 5, control of the furnace atmosphere in accordancewith automatic and continuous de termination of the composition of the furnace at mosphere provides for steady, efficient operationof the furnace at its maximum capacity;

Although the present inventionv hasbeen described in conjunctionwithcertain specific .ems. bodirnents thereof, as thoseskilled in the artrwill readily understand; variations and modifications thereof can be made. Such variations and modifications are to be considered withinthepurview of the specificationand the: scope of theappendedclaims. We claim: a i

1. A method of operating melting,-reducing furnaces which comprises charging a rnixtureof: nickel oxide and azcarbonaceousz reducing agentmatic and continuous sampling.of thefurnaceiatmosphere after removing the carbon dioxide from said atmosphere, and automatically recording the hydrogen content of said carbon dioxide-free atmosphere by a specific gravity measuring instrument.

2. A method for operating melting-reducing furnaces of the open hearth type which comprises charging into said furnace a mixture of nickel oxide and a carbonaceous reducing agent, heating said charge to reaction temperatures by means of a fuel of substantially constant composition, maintaining a reducing atmosphere containing hydrogen and carbon dioxide in said furnace by controlling the hydrogen content of said furnace atmosphere within the limits of about 6% to about 9 at the time when the charge is first heated and decreasing as the time of heating increases to a hydrogen content of about 1.8%. to about 4% at the end of 7 hours of operation, continuing to reduce the hydrogen content of said furnace atmosphere during further operation to about 1.2% to about 3.5%, controllingsaid hydrogen content of said atmosphere by automatic and continuous sampling of the furnace atmosphere after removing the carbon dioxide from said atmosphere, and automatically recording the hydrogen content of said carbon dioxide-free atmosphere by a specific gravity measuring instrument.

-3. A method for operating melting-reducing furnaces of the open hearth type which comprises introducing a charge comprising nickel oxide and a carbonaceous reducing agent into a furnace, heating said charge to reaction temperatures by means of a carbonaceous fuel of substantially constant composition, regulating the furnace atmosphere containing hydrogen and carbon dioxide to provide a reducing atmosphere having controlled hydrogen content dependent upon the time of heating, maintaining the furnace atmosphere to contain hydrogen in amounts of about 6% to about 9 /2% at the beginning of the operation, decreasing the hydrogen content to about 1.8% to about 4% at the-end of '7 hours of heating, further decreasing the hydrogen content to about 1.2% to about 3.5% at the end of 9 hours of heating and bein maintained at about 1.2% to about 3.5%- during the next 7 hours, controlling the hydrogen content of said furnace atmosphere by automatic and continuous sampling of the furnace atmosphere after removing the carbon dioxide from said atmosphere, and automatically recording the hydrogen content of said carbon dioxide-free atmosphere by a specific gravity measuring instrument.

4. The method of reducing nickel oxide and meltin'g'the nickel which comprises mixing nickel oxide with a carbonaceous reducing agent to provide a reaction mixture, heating said reaction mixture in a reducing atmosphere containing carbon dioxide and having a controlled hydrogen content dependent upon the time of heating at temperatures at which reduction takes place, controlling the hydrogen content of said reducing atmosphere in accordance with the composition of said furnace atmosphere determined continuously and automatically to provide a furnace atmosphere having a hydrogen content of about 6% 'to about il when heating begins, decreasing to about 1.8% to about 4% at the end of about 7 hours, continuing to decrease to about 1.2% to about 3.5% at the end of 9 hours and being maintained at about 1.2% to about 3.5%

during the ensuing 7 hours, controlling the bydrogen content of said furnace atmosphere by automatic and continuous'sampling of the furnace atmosphere after removing the carbon dioxide therefrom, and automatically recording the hydrogen content of said carbon-dioxide-free atmosphere by a specific gravity measuring instrument.

5. A method for reducing nickel oxide and melting the nickel which comprises mixing nickel oxide with coke to form a reaction mixture, heating said reaction mixture in a reducing atmosphere containing carbon dioxide and having a controlled hydrogen content, the hydrogen content of said furnace atmosphere being regulated in accordance with the time of heating to contain a predetermined amount of hydrogen between about 6% and about 9 /2% at the start of the heating, decreasing the hydrogen content to about 1.8%'to about 4% at the end of 7 hours, continuing to decrease to about 1.2% to about 3.5% at the endof 9 hours and being maintained at about 1.2% to about 3.5% for the next 7 hours, regulating-the fuel-air ratio whereby said charge is heated by automatically and continuously sampling the furnace atmosphere after removing the carbon dioxide therefrom, and automatically recordingthe hydrogen content ofv said carbon dioxide-free atmosphere by a specific gravity measuring instrument.

6. The method of operating a reduction-melting furnace which comprises mixing nickel oxide with a carbonaceous reducing agent to form a reaction mixture, heating said reaction mixture to reaction temperature, automatically and continuously sampling the furnace atmosphere, which contains hydrogen and carbon dioxide, drying-the samples, passing said dried samples through an absorbent for carbon dioxide to remove the carbon dioxide from said furnace atmosphere,,determining the specific-gravity of said samples and automatically and continuously comparing the specific gravity of said carbon dioxide-free samples with the specific gravity of a standard gas under standard conditions of humidity and temperature; registering thereiative .sp ecific gravity of said carbon-dioxide-free samples as p'er cent hydrogen and regulating the fuel-air ratio in accordance with said hydrogen content to maintain a furnace atmosphere containing an amount of hydrogen of about 6% to about 9 when heatinghas begun, decreasing to about 1.8% to about 4% at the end ofabout 7 hours, continuing to decrease to about 1.2% to about 3.6% at the end of 9 hours and continuing at about 1.2% to about 3.6% during theinext'ihours.

'7. Themethod for operating melting-reducing furnaces by continuously sampling a furnace atmosphere containing 002,00, H2, N2, and H20 and-recordingjthe hydrogen content of said atmosphere, which comprises heating a charge containing nickel oxide and a carbonaceous reducing agent in:a melting-reducing furnace having said atmosphere, withdrawing a sample of said atmosphere from said furnace, drying said sample, removing CO2 from said dried sample, comparing the specific gravity of said dried CO2- free'sample of furnace gas with a standard gas under regulated conditions of temperature and humidity to determine the per cent hydrogen present in said sample, recording the percent hydrogen in said sample, and operating said furnace in accordance with said recordings.

- JOSEPH EDWIN CARTER.

RAY KERMIT GENSLER. 

